Method of eliminating stress corrosion cracking in copper-magnesium-zinc series aluminum alloys



United States Patent California No Drawing. Filed Nov. 25, 1964, Ser. No. 414,875 7 Claims. (Cl. 148-159) This invention relates to high strength aluminum alloys of the Copper-Magnesium-Zinc Series and to a method of making the same, and more particularly to high strength aluminum alloys in which the physical properties of the alloys are held at the highest possible level while substantially eliminating any structural failure of the alloys caused by Stress Corrosion Cracking.

Susceptibility to stress corrosion cracking is by no means peculiar to alloys of aluminum since it is well known in the brasses, stainless steels and numerous other alloys. This phenomenon has been the subject of intensive investigation and is generally attributed to the simultaneous presence of stress and a precipitated constituent within the alloy which by the nature of its mode of formation and chemical composition is cathodic to the impoverished areas of the solid solution caused by the formation of said precipitate. As a consequence chemical attack takes place on this area when the alloy is exposed to or immersed in a corroding medium. The chemical attack is greatly increased by stress across the contact between the precipitate and the adjacent material, and this stress tends to rapidily open up crevices formed by the initial chemical attack, and the resulting failure is sudden and dramatic. The anodic behavior of the ground-mass is generally attributed to local impoverishment in the elements making up the precipitate and arises from the diffusion of these elements out of the lattice-Work of the solid solution in forming the precipitated particle. V

The phenomenon Stress Corrosion Cracking is related to tergranular Corrosion which is caused by similar conditions and proceeds in a similar manner but is not generally as subject to acceleration of attack and is not dependent upon the presence of stress although it may be influenced by stress. Neither does.failure in intergranular corrosion result in dramatically sudden failure so generally characteristic of stress corrosion cracking. In either case, however, the incidence of stress corrosion cracking and intergranular corrosion can often be eliminated, or at least greatly reduced, by relief of the stress and by causing diffusion to takeplace in the solid solution within the body of the grains of the alloy, thereby reducing its anodic behavior.

Heretofore, attempts to eliminate stress corrosion cracking have been unsuccessful because these eiforts also resulted in drastic reduction of the strength levels and thus reduced the practical elfectiveness of the treatment for these high strength aluminum alloys.

An object of this invention is to preserve a high strength level in aluminum alloys of the copper-magnesium-zinc series while simultaneously eliminating stress corrosion cracking susceptibility. Thus Weight penalties incurred by ordinary design criteria taking into account the usu susceptibilities are thereby avoided.

3,231,435 Patented Jan. 25, 1966 Another object of this invention is to reduce the anodic nature of the areas surrounding the precipitated constituents within the aluminum alloy, an alloy of the coppermagnesium-zinc series, in which the precipitate by the nature of its mode of formation and chemical composition is cathodic to the solid solution from which it is formed. That is, We are altering the character of the base material surrounding the precipitate. As a result of the treatment that We provide, chemical attack is not accelerated upon the base material when the alloy is exposed to or immersed in a corroding medium, nor does imposed stress cause corrosive attack as is usually found at the grain boundaries.

It is also an object of this invention to provide a simple and feasible method step and/or steps which can be accepted by those skilled in the art and carried out with exactly determinable results. A further related object is to provide an aluminum alloy, treated according to our invention, which exhibits physical properties not found in aluminum alloy heat treated by other methods, in addition to the retention of approximately of the high strength properties typical of T6 temper treated alloys.

The various objects and features ofthis invention will be fully understood from the following detailed description of the typical preferred forms and applications of the method, and from the tables included and referred to therein.

Aluminum alloys of the copper-magnesium-zinc series are known as high strength aluminum alloys, and the composition of the two most commonly used alloys in the aluminum alloy series under consideration are listed below in Table 1. Generally, it has been found that the higher the alloy content, the higher are the strengths attained after the standard heat treatment known in the industry as-the T6 temper treatment and described more fully hereinafter. Table II lists the typical tensile properties usually attained by forging. of these two common alloys after .the T6 temper treatment. The alloys are listed in these tables by the designation number allocated by the Aluminum Association and by which the particular alloy is known to the aluminum industry of today.

TABLE I Typical composition of high strength aluminum alloys (COPPER-MAGNESIUM-ZINC SERIES) Alloy Mg Zn Cu Or TABLE II Typical tensile properties of high strength aluminum alloys-forgings (COPPER-MAGNESIUM-ZINC SERIES) The above high strength aluminum alloys are widely used in applications subjected to high stresses and provide a means of producing li ht weight structures. The said aluminum alloys in this composition system are, however, susceptible to stress corrosion cracking which becomes particularly evident and important in heavy sections stressed in the short transverse direction (the direction parallel to the thickness of the piece and perpendicular to the direction of working). This susceptibility increases with the alloy content, and with the high strength characteristics, and has consequently thwarted the development of the said high strength aluminum alloys in the magnesium-zinc series. That is, the combined effect of high alloy content and high strength in increasing the susceptibility has resulted in limiting the usable maximum strength level in engineering applications.

When stresses in the short transverse direction are encountered by the structure or assembly it becomes necessary to increase the section in order-to arrive at a low stress level, and this requirement for increase results in significant weight penalties. Structures in which stress corrosion is important and which are made from forgings of the 7075-T6 alloy for example, with a guaranteed longitudinal yield strength of 65,000 p.s.i., will be designed for a maximum stress of 50,000 p.s.i. in the longitudinal direction, 30,000 psi. in the long transverse direction and 7,000 psi. in the short transverse direction. This 7,000 psi. stands in strong contrast to the guaran. teed 65,000 psi. yield strength. The weight penalties incurred by the necessity of increasing the section to safely carry the required loads can be very significant, and it is clear that any decrease that can be made in the stress corrosion cracking susceptibility of these aluminum alloys has great utility.

Improvement of these aluminum alloys is usually at tempted by aging the alloy at higher than normal temperatures or by reheating normally aged material to reduce residual stresses and thereby allow diffusion to take place, thus reducing the anodic character of the solid solution. Sufiicient time must be allowed for the diffusion to take place and to the degree necessary to reach a state of uniform concentration. In high strength aluminum alloys the temperature and time required to eliect these results have been too great and have resulted in drastic losses of strength, usu zgly to a degree which will defeat the purpose of the treatment. However, we provide a method of treating these alloys which will, for all practical purposes, eliminate the said susceptibility to stress corrosion cracking or the like, and at the same time preserve a high strength level in the said aluminum alloys. The method hereinafter disclosed is elTective for the alloys of Table I despite their inherent tendency for greater susceptibility with increasing alloy content.

The susceptibilities of the said alloys to stress corrosion cracking when heat treated by standard practices to properties typical of the alloy in the solution treated and artificially aged condition, known to the industry as the T6 temper, are set forth in Table III. It will be noted there that the higher the applied stress the greater the susceptibility.

TABLE III Susceptibility of alloys to stress corrosion cracking DAYS TO FAILURE INDICATED STRESS LEVELS The specimens in Table III are taken from forgings and stressed in the short transverse direction. The time intervals recorded are the mid-point of the range of times to failure. The nature of the testing is such that when a series of specimens are tested at a given level some will fail sooner than others and, generally speaking, the lower the stress the longer the average survival time. From a practical viewpoint, the useful stress for the material is the stress level at which it exhibits no failure during the test period. As Table III shows, this useful stress level is in the order of 7,000 psi. for this material when stressed in the short transverse direction.

When these aluminum alloy materials are tested with the stress in the longitudinal, or in the long transverse direction, the general pattern is much the same but average survival time for a given stress level is considerably increased and stress at which failure will not occur is also increased. The longitudinal direction shows the least susceptibility to this type of failure, while the short transverse direction shows the greatest susceptibility. The

susceptibility in the said short transverse direction is therefore used as the test criteria.

In the copper-magnesium-zinc series of aluminum alloys we have discovered that stress corrosion susceptibility can be eliminated by means of a specific method of heat treatment. We have also discovered that by this treatment at least eifectiveness of the high strength properties typical of the T6 condition in these alloys can be retained while thus eliminating said susceptibility to stress corrosion cracking. The temperatures and times employed in the standard T6 temper heat treatment used by the aluminum industry for these high strength aluminum alloys are shown in Table IV, below:

- TABLE IV Standard heat treatment temperatures-T6 temper (COPPE R-MA GNE SIUM'ZINC SE RIE S) Solution Aging Aging Alloy Tempera- Ternpera- Time, hrs.

ture, F. ture, F.

* Followed by quenching in water.

In accordance with the present invention the method that we have provided is applied to the aluminum alloy in the solution treated condition; that is, after a controlled water quench from 860 to 890 F. The method consists of artificially aging the material at 205 to 225 F. for six to ten hours instead of the usual and standard 230 to 260 F. for approximately twenty-two to twenty-four hours, and then with continuous heating, raising the temperature to 300 to 380 F. for two to forty-eight hours and then air cooling in the normal fashion. The significant step for the elimination of stress corrosion cracking is the separate and supplemental heating to the 300 to 380 F. temperature for two to forty-eight hours. The lower temperatures in this specified separate and supplemental heating range will require the longer time interval in order to accomplish the beneficial effects and the higher temperatures will require the shorter time interval. Similarly, exposure to the'lower temperatures for too short a time will not completely eliminate susceptibility to stress corrosion cracking while exposure to the higher temperature for too long a time will bring about excessive lowering of the strength. In actual practice, the preferred ranges of temperature and time which will preserve the maximum degree of strength and which will also eliminate susceptibility to stress corrosion cracking is the tem-' perature range of 315 to 355 F. for the time periods of four to eighteen hours. We have found that the application of the lower temperatures of 205 to 225 F. and the shorter time of siX to ten hours for the artificial aging and the continuity of heating to the supplementary temperatures of 300 to 380 F. or 315 to 355 F. contributes to retaining of at least 80% of the T6 temper properties.

Our method may be varied by cooling to room temperature after the artificial aging in the 205 to 225 F. temperature range for six to ten hours and then reheat ing to the said separate and supplemental temperature range of 300 to 380 F. for two to forty-eight hours. This, however, results in tensile properties being lowered by 2 to 5% below those obtainable with the preferred continuous form of cmrying out the process as above described.

Another variation of our method involves the said separate and supplemental heating to a temperature range of 300 to 380 F. for two to forty-eight hours which is applied to material cooled to room temperature after being artificially aged at the standard temperature (230 to 260 F.) for the standard or usual time of twenty-two hours. This variation of our method also eliminates the stress corrosion susceptibility of the alloys; however, again the tensile properties of the alloys are lowered by 2 to 5% below those obtainable with the above-described preferred continuous form of our invention.

In still another variation our method involves the said separate and supplementary heating to a temperature range of 300 to 380 P. which is applied to material artificially aged at the standard temperature of 230 to 260 F. for the usual and standard time of twenty-two hours without cooling to room temperature before the said separate and supplementary heating to 300 to 380 F. for two to forty-eight hours. Again, stress corrosion cracking susceptibility will be eliminated but the tensile properties may be 2 to 5% lower than with our first described and preferred form of carrying out the method.

Comparisons of the properties otbained by the preferred form of our heat treatment method with those typical of the same and usual heat treated alloys in the T6 condition are shown in the following two examples:

Example I.A lot of 7075 alloy forging tested in the T6 condition and also tested after being given the heat treatment of the present invention:

TENSILE PROPERTIES SUSCEPTIBllITY TO STRESS CORROSION CRACKING Maximum number of days to failare5 specimens Stress Level, p.s.i. T6 Condition After Our Treatment 1 None in 112 da.

Example II.A lot of 7001 aluminum alloy forgings tested in the T6 condition and also tested after being given the heat treatment of the present invention:

TENSILE PROPERTIES SUSCEPTIBILITY TO STRESS CORROSION CRACKING Maximum days to failure-5 specimens Stress level, p.s.i. T6 Condition After Our Treatment None after da.

For the purposes of our invention, a relatively rapid water quench rate from the solution temperature (860- 890 F.) is important to insure the attainment of the desired mechanical properties. Such a relatively rapid quench rate may be defined as any quenching method which will produce properties, including resistance to corrosion, equivalent or superior to those produced by a standard quench in water at room temperature and which will, at the same time, produce an alloy susceptible to stress corrosion cracking. Of course it is recognized that this rapid quenching rate may cause the alloy to become distorted or warped; however, this invention is concerned with the elimination of stress corrosion cracking and not the avoidance of distortion or warpage.

The above definition is designed to encompass a wide range of quenching rates, including quenching in cold and warm water, and also including industry and government specification controlled quench rates which when properly performed do not render excessive intergranular corrosion in unstressed alloy material exposed to standard corrosion mediums.

The normal quenching methods used in the industry for aluminum alloys concerned in our invention utilize cold and warm water, and in comparative tests between samples of the same alloys treated according to our invention and according to the above-described T6 temper heat treatment, we have used both cold water and F. water quenches with essentially similar results, as shown below in Table V. It should be noted that in each case, regardless of whether cold or warm water Was used to quench, the samples treated according to the T6 temper method were susceptible to stress corrosion cracking, as evidenced by the short life in the standard 30-day alternate immersion test. In contrast to this, the samples treated according to this invention were not susceptible to stress corrosion cracking as shown by sample life in excess of 100 days under the same conditions in the standard 30-day alternate immersion test.

TABLE V Physical properties and susceptibility to stress corrosion cracking after cold water and 150 F. water quenches Aging Treat- Tensile Yield Percent Life In Stress Alloy ment Strength Strength Elongation Corrosion Test Cold Water Quench 7001 T6 Temper 105, 800 00, 300 10. 1-4 Days. 7001 Our Treatment. 80, 200 70, 700 12.0 More than 100 Days.

150 Water Quench 7075 T6 Temper" 89, 600 82, 700 11.0 4-6 Days. 7075 Our Treatment- 73, 200 64, 400 12. 5 Mpjre than 100 ays. 7001. T6 Tempen- 105, 100 98, 900 10. 0 4 Days. 7001- Our Treatment. 78, 000 G8, 900 10. 5 MoDre than 100 ays. 700l I d0 85,800 78, 700 10.5 Do.

From the foregoing it will be apparent that the additional and supplemental heat treatment step conducted within the range of temperatures and time intervals as specified is effective in eliminating stress corrosion cracking or the like in high strength aluminum alloys of the copper-magnesium-zinc series, and that the strength levels contained are approximately 80% of the usual or standard T6 condition levels therefor. The said additional or supplemental step of the method is simple and effective and can be carried out with existing equipment used in the usual heat treating of said alloys to the said T6 condition. Although this susceptibility prevention method is operable within the aforementioned heat and time interval ranges, it is to be undrestood that application of heat and time can be augmented or diminished as circumstances require. Heat in excess of 380 F. results in additional strength losses. A lesser time interval than 2 hours results in lesser effectby the method step, and conversely a greater time interval than 48 hours results in strength losses. Therefore, as determined by actual application of the method to said aluminum alloys, the preferred and apparently most practical range of temperature and time for the specific alloys mentioned herein is 315 F. to 355 F. and for time periods of 4-18 hours. By confinement ofthe said additional heat treating step, as hereinabove disclosed, to the preferred ranges set forth, strength is retained in the said alloys and susceptibility to said failure is eliminated.

Previous attempts to eliminate stress corrosion cracking in these high strength aluminum alloys have been unsuccessful apparently because the temperatures and heating times utilized were excessive. These efforts were also aimed primarily at relief of residual stresses rather than at effecting controlled local diffusion in the solid solution area near and surrounding the precipitating constituents, and these efforts have reduced the strength characteristics. In our process, as hereinabove set forth, we allow the precipitation to take place at the temperatures which produce the optimum properties, and after these are established We reheat the material within the above prescribed time and temperature range. This treatment causes selective diffusion of the alloying constituents into the impoverished areas surrounding the precipitate thereby reducing the anodic character of these areas without permitting excessive particle growth, and hence the reduction in strength is controlled.

Moreover, it is true, of course, that our novel method of heat treating high strength aluminum alloys is not the first process wherein an alloy is age hardened at two different temperatures. The British Patent No. 544,439, granted to the Aluminum Company of America (complete specification accepted April 14, 1942), discloses a method for improving the corrosion resistance of aluminum base alloys which had been subjected to a solution heat treating by giving the alloys 2. further, comparatively low temperature heat treatment following the normal artificial age hardening. However, the process disclosed in this British patent is distinguishable in that the patent specifically states that the alloy is a relatively slowly quenched (by definition, more slowly than a water quench) so as to avoid completely any distortion or warping normally caused by the relatively rapidly quenching solution heat treated alloys. As a consequence of this relatively slow quenching, the age hardened alloys treated according to the disclosure in the British patent have a stress corrosion resistance which is only equivalent to that found in alloys which had been rapidly quenched from the solution heat treatment condition and age harden in the normal manner, i.e., according to the T6 temper treatment. Thus it is apparent that the British patent is mainly concerned with providing a distortionfree aluminum alloy and is completely satisfied when the alloys have stress corrosion resistant properties equivalent to alloys which have been treated in the normal manner. Also, it should be noted that the process disclosed in the British patent has been practiced in the United States for at least twenty years and in no instance has it been found to eliminate stress corrosion cracking.

In contradistinction to this, aluminum alloys heat treated according to our invention have stress corrosion resistant properties which are significantly superior to those found in alloys treated in the normal manner, i.e., the T6 temper process, and thus alloys treated according to the disclosure in the British patent. This is shown by the fact that stress corrosion cracking is, for all prac tical purposes, completely eliminated in alloys treated according to our invention.

Moreover, as noted hereinabove, it was found that the high strength aluminum alloys treated according to our invention have certain improved physical properties, in addition to freedom from corrosion stress cracking. These advantageous properties include significantly increased electrical conductivity, a marked less susceptibility to selective grain etching and less grain contrast as indicated by a more matted structure of the alloy. These properties are not found in alloys treated according to the T6 temper process or the process disclosed in the above-mentioned British Patent No. 544,439 and are considered to be significant because they greatly increase the versatility of the alloys, as well as permitting the alloys to perform more satisfactorily in their present applications.

Furthermore, the hereina-bove described process becomes of greater utility when applied to the alloys in this series having the higher alloy content. It is these alloys that exhibit the highest strengths which concurrently show the greatest susceptibility to stress corrosion cracking.

This is evident in a comparison of Examples 1 and 2. In Example 1, the 7075 alloy, as compared to the 7001 alloy of Example 2, has the lower alloy content, the lower strentgh and the lower susceptibility to stress corrosion cracking in the T6 condition. The stress corrosion cracking susceptibility is, however, eliminated in both alloys by our treatment and the 7001 alloy having the higher alloy content retains a higher strength level. We have found that conducting the treatment in a continuous transition manner affords the greatest strength retention, and the data presented herein is representative of this preferred form of our method.

Having described only the typical preferred forms and applications of our method, we do not wish to be limited or restricted to the specific details herein set forth, but wish to reserve to ourselves any modifications or variations that may appear to those skilled in the art and fall within the scope of the following claims.

We claim:

1. A process for heat treating a high strength aluminum base alloy of the copper-magnesium-zinc series whereby the alloy is rendered substantially nonsusceptible to stress corrosion cracking while retaining its high strength properties, the process including: heating the alloy at a temperature of between 860 F. and 890 F. for a time sufiicient to allow constituents of the alloy to enter into solid solution; water quenching the alloy; aging the alloy by heating to a temperature of between 205 F. and 225 F. for six to ten hours; and thereafter heating the all-y to a temperature of between 300 F. and 380 F. for two to forty-eight hours.

2. A process for heat treating a high strength aluminum base alloy of the copper-magnesium-zinc series whereby the alloy is rendered substantially nonsusceptible to stress corrosion cracking while retaining its high strength properties, the process including: heating the alloy at a temperature of between 860 F. and 890 F. for a time suflicient to allow constituents of the alloy to enter into solid solution; water quenching the alloy; aging the alloy by heating to a temperature of between 205 F. and 225 F. for six to ten hours; cooling the alloy to room temperature and thereafter reheating the alloy to a temperature of between 300 F. and 380 F. for two to forty'eight hours.

3. A process for heat treating a high strength aluminum base alloy of the copper-magnesium-zinc series whereby the alloy is rendered substantially nonsusceptible to stress corrosion cracking while retaining no less than approximately 80% of its high strength properties, the process including: heating the alloy at a temperature of between 860 F. and 890 F for a time suflicient to allow constituents of the alloy to enter into solid solution; water quenching the alloy; aging the alloy by heating to a temperature of between 205 F. and

10 225 F. for six to ten hours; and thereafter heating the alloy to a temperature of between 315 F. and 355 F. for four to eighteen hours.

4. A process for heat treating a high strength aluminum base alloy of the copper-magnesium-zinc series whereby the alloy is rendered substantially nonsusceptible to stress corrosion cracking while retaining its high strength properties, the process including: heating the alloy at a temperature of between 860 F. and 890 F. for a time sufiicient to allow constituents of the alloy to enter into solid solution; water quenching the alloy; aging the alloy by heating to a temperature of between 230 F. and 260 F. for approximately 22 hours; cooling the alloy to room temperature and reheating the alloy to a temperature range of between 300 F and 380 F. for two to forty-eight hours.

5. A process for heat treating a high strength aluminum base alloy of the copper-magnesium-zinc series whereby the alloy is rendered substantially nonsusceptible to stress corrosion cracking while retaining no less than approximately of its high strength properties, the process including: heating the alloy at a temperature of between 860 F and 890 F. for a time sufiicient to allow constituents of the alloy to enter into solid solution; water quenching the alloy; aging the alloy by heating to a temperature of between 205 F. and 225 F. for six to ten hours; cooling the alloy to room temperature and thereafter reheating the all-0y to a temperature of betweeen 315 F. and 355 F. for four to eighteen hours.

6. A process for heat treating a high strength aluminum base alloy of the copper-magnesium-zinc series whereby the alloy is rendered substantially nonsusceptible to stress corrosion cracking while retaining no less than approximately 80% of its high strength properties, the process including: heating the alloy at a temperature of between 860 F. and 890 F. for a time suflicient to allow constituents of the alloy to enter into solid solution; water quenching the alloy; aging the alloy -by heating to a temperature of between 230 F. and 260 F. for approximately 22 hours; cooling the alloy to room temperature and reheating the alloy to a temperature range of between 315 F. and 355 F. for four to eighteen hours.

7. A high strength copper-magnesium-zinc aluminum alloy heat treated according to the process described in claim 1.

References Cited by the Examiner FOREIGN PATENTS 544,439 4/ 1942 Great Britain.

DAVID L. RECK, Primary Examiner. 

1. A PROCESS FOR HEAT TREATING A HIGH STRENGTH ALUMINUM BASE ALLOY OF THE COPPER-MAGNESIUM-ZINC SERIES WHEREBY THE ALLOY IS RENDERED SUBSTANTIALLY NONSUSCEPTIBLE TO STRESS CORROSION CRACKING WHILE RETAINING ITS HIGH STRENGTH PROPERTIES, THE PROCESS INCLUDING: HEATING THE ALLOY AT A TEMPERATURE OF BETWEEN 860*F. AND 890*F. FOR A TIME SUFFICIENT TO ALLOY CONSTITUENTS OF THE ALLOY TO ENTER INTO SOLID SOLUTION; WATER QUENCHING THE ALLOY; AGING THE ALLOY BY HEATING TO A TEMPERATURE OF BETWEEN 205*F. AND 225*F. FOR SIX TO TEN HOURS; AND THEREAFTER HEATING THE ALLOY TO A TEMPERATURE OF BETWEEN 300*F. AND 380*F. FOR TWO TO FORTY-EIGHT HOURS. 